Non-metallic sheathed cable, such as NM-B type cable, is often used for providing electrical systems within residential structures. Known non-metallic sheathed cable assemblies often comprise one or more electrical conductors individually coated in an electrical insulator (e.g., a solid or stranded copper wire coated in a plastic material) bundled together and collectively sheathed in a non-metallic outer sheath. Generally the non-metallic outer sheath comprises a non-conductive polymer such as poly-vinyl chloride (PVC) and has been understood to provide mechanical protection for the bundled wires against insulation tears and abrasion.
During installation, these cables often must be threaded through a series of rough-hewn holes cut through wooden floor and ceiling joists, headers, and wall studs (such as those commonly referred to as 2×4s, 2×6s, 2×8s, and/or the like) or through narrow plastic conduit. Due to time pressures involved in installing electrical cable and the often complex shapes of walls and structures included in residential buildings, the electrical cable installation path comprises a substantially non-linear path through multiple wooden studs. These cable installation paths often run substantially horizontally through a series of wooden wall studs, turn around corners to follow additional path segments substantially perpendicular to previous segments, and turn vertically to run along the length of wooden wall studs to electrical outlets, wall mounted switches, or ceiling mounted light sources.
The exterior surfaces of the non-metallic sheathed cables often have a high dynamic coefficient of friction, and therefore installation of these cables along the installation path may require a substantial pull force to overcome the frictional force occurring between the non-metallic sheathed cable and the surfaces of the installation path while the cable is being pulled. In some installations, the pulling force necessary to move the cable through the installation path may be high enough to deform or tear the outer non-metallic sheath. Therefore, when installing long segments of non-metallic sheathed cable, multiple installers may be necessary to thread the non-metallic sheathed cable along the installation path. A first installer may be necessary to push lengths of cable into the installation path, and one or more additional installers may be necessary to pull the provided lengths of cable along the installation path.
Electricians and installers have previously coated non-metallic sheathed cables with a separate cable lubricant, often in the form of a liquid or gel, at the installation site to reduce the coefficient of friction of non-metallic sheathed cables. Applying these separate cable lubricants at the job site may require additional installation time, can be messy, and, depending on the chemical composition of the lubricant, may negatively impact the mechanical and insulative properties of the non-metallic sheath.
As noted in U.S. Pat. No. 7,411,129 to Kummer et al., incorporated herein by reference, and patents and patent applications related thereto, advances have been made in decreasing the pulling force necessary to install electrical conductors in substantially non-linear installation paths. These advances include sheath formulations wherein nylon or another polymer is mixed with a lubricant and formed over the outside of the conductor in order to decrease the surface coefficient of friction. However, these efforts have been directed to generally circular conductors, such as circular Thermoplastic High Heat Resistant Nylon (THHN) wiring or the like.
In many residential installations, however, substantially flat or non-circular cables, such as Southwire's Romex® 14/2, 12/2, or 10/2 cable, are used for a substantial portion of electrical wiring. Such cable may comprise two separately insulated conductors and a separate ground wire arranged in a substantially flat arrangement (e.g., the center point of each of the three wires are nominally aligned in a single plane). The three wires are encapsulated in a non-metallic outer sheath as described above. As noted herein, previous attempts to decrease the pulling force necessary for installation of non-metallic sheathed cables have been limited to circular wires and cables. Physical characteristics of the materials utilized in the reduction of the surface coefficient of friction for non-metallic sheathed cables have previously limited the commercial manufacture of previously known methods and materials to circular cables and wires. Moreover, product safety and certification organizations, such as the Underwriters Laboratory (UL), have implemented sheath thickness and uniformity standards for non-metallic sheathed cables, highlighting the importance of a uniform sheath thickness. Therefore, new manufacturing methods are needed to consistently produce non-circular, non-metallic sheathed cable with a reduced surface coefficient of friction in order to decrease the pull force necessary to install these cables in generally non-linear cable installation paths.